(1) Field of the Invention
The present invention relates generally to marine vehicle maneuverability, and more particularly to a method and system for improving the maneuverability of a marine vehicle equipped with a lateral tunnel thruster by controlling the pitch angle of a tunnel thruster""s propeller.
(2) Description of the Prior Art
Marine vehicles typically use rudders or other control surfaces to produce maneuvering forces. However, flow over the control surfaces is required to produce a maneuvering force that varies with the square of the vehicle speed. Therefore, at low speed, control surfaces become ineffective. To combat this problem, tunnel thrusters have been located in the bow or stern of marine vehicles to meet lateral low speed maneuvering requirements.
In general, a lateral tunnel thruster has a rotating propeller installed in a laterally traversing tunnel extending through the vehicle. The rotating propeller creates a pressure differential across the blades and drives a jet of water through the tunnel and out one side. The integrated pressure force on the blades is transferred to the vehicle as a force acting in the opposite direction of the jet flow which, in turn, is used to maneuver the vehicle. For most applications, lateral tunnel thrusters are designed to be reversible so that the vehicle may be maneuvered in either port or starboard directions.
Unfortunately, the effectiveness of tunnel thrusters decreases with forward velocity of the vehicle. Further, there is often an intermediate vehicle speed at which neither the control surfaces nor the thruster produce effective maneuvering forces. Studies that measure the effects of forward vehicle velocity on tunnel thrust performance show that as the forward velocity increases, the effective side force (i.e., force perpendicular to the vehicle axis) from the tunnel thruster decreases to as low as ten percent of the side force measured at zero forward vehicle velocity. In other words, tunnel thrusters quickly lose their maneuvering effectiveness as forward vehicle velocity increases.
Experiments conducted to understand this phenomenon indicate that the forward velocity on the vehicle significantly increases fluid velocity through the tunnel for a fixed rotor speed. This results in the propeller blade operating off design and unloading the blades which results in less thrust on the vehicle.
Accordingly, it is an object of the present invention to provide a method and system that improves the maneuverability of a marine vehicle.
Another object of the present invention is to provide a method and system for improving the efficiency of a marine vehicle""s lateral tunnel thruster.
Still another object of the present invention is to provide a method and system for improving the performance of a marine vehicle""s lateral tunnel thruster regardless of the forward vehicle speed.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system is provided that improves the maneuverability of a marine vehicle. The marine vehicle is one that has a lateral tunnel in which a propeller is mounted such that, when the marine vehicle moves at a forward speed through water, a water flow having a fluid velocity moves through the lateral tunnel. The propeller is turned at a substantially constant rate of rotation and the fluid velocity in the tunnel is measured either directly or indirectly as a function of the vehicle""s forward speed. The pitch angle of the propeller is adjusted based on the measured fluid velocity.